I just downloaded some e-books via file sharing client and I want to know if they are real or fake. I mean could it possible that the content in some pages may be altered? Is there any guarantee about it? Do I need to worry about it at all? Surely, you guys may have more experience about downloading such things.

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There is really know way to know if they are altered rather than comparing to a known good version. The old adage applies...you get what you pay for.
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EBGreenOct 22 '12 at 14:46

But I have heard some people read them anyway. Would you guys too without knowing the source?
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user1210321Oct 22 '12 at 14:57

1 Answer
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The only way you could verify the contents would be a direct comparison to a known good version of the work. The other option is that if the file is an exact copy of a specific file (supposedly), you can review the checksum, commonly done using md5sum. This only works if the two files should be identical to each other. If you had that luxury, you would not be questioning the authenticity to begin with.

If you only have a single copy of a work, then you have nothing to compare it against. An e-book is just a (usually) read only document formatted for a particular type of software package. The contents it holds is arbitrary and anybody can create an e-book that says anything, just like they could with a standard book.

In short, without a known good copy to compare it against, you have no real way to verify its accuracy to the original.

hash comparison would generally not be very effective. Most such PDFs are generated by scanning the book then running the scan through OCR software. Often as one process. The variation in OCR software and the scans means that you will very often see PDFs that substantively both accurate but that would have very subtle differences that would show them as different to hash comparison.
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EBGreenOct 22 '12 at 15:11

Correct, that is the reason for my caveat that it only will work if it is an identical file to a known good copy.
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UtahJarheadOct 22 '12 at 15:15